Field of the Invention
The present invention relates to a sputtering target for use in forming a Cu—In—Ga—Se compound film (hereinafter abbreviated as “CIGS film”) for forming a light absorbing layer of a thin-film solar cell, and a method for producing the same.
Description of the Related Art
In recent years, thin film solar cells made by using a chalcopyrite compound semiconductor have been practically employed. The thin-film solar cell made by using the compound semiconductor has a basic structure in which a Mo electrode layer serving as a positive electrode is formed on a sodalime glass substrate, a light absorbing layer consisting of a CIGS film is formed on the Mo electrode layer, a buffer layer consisting of ZnS, CdS, and the like is formed on the light absorbing layer, and a transparent electrode layer serving as a negative electrode is formed on the buffer layer.
As a method for forming the light absorbing layer described above, a method for depositing a film by vapor deposition is known. Although a light absorbing layer obtained by the method may exhibit high energy conversion efficiency, film deposition by vapor deposition attains slow deposition rate. Hence, when a film is deposited on a substrate having a large area, the uniformity of the in-plane distribution of the film thickness is readily reduced. Thus, a sputtering method for forming a light absorbing layer has been proposed.
As a sputtering method for forming the light absorbing layer, a method (so called “selenization method”) has been proposed in which an In film is firstly deposited by sputtering using an In target, a Cu—Ga binary alloy film is deposited on the In film by sputtering using a Cu—Ga binary alloy target, and a stacked precursor film consisting of the obtained in film and the Cu—Ga binary alloy film is subject to heat treatment in a Selenium atmosphere to thereby form a CIGS film.
In order to improve the power generation efficiency of a light absorbing layer consisting of a CIGS film, the addition of Na to the light absorbing layer by diffusion from the alkaline glass substrate is known as effective means as disclosed in, for example, Non-Patent Document 1. However, in the case of a flexible CIGS solar cell based on a polymer film or the like instead of an alkaline glass, a supply of Na may undesirably be lost because there is no alkaline glass substrate.
Thus, for example, Patent Document 1 proposes a diffusion of Na from a lift-off layer to a light absorbing layer by providing the lift-off layer of sodium chloride in order to improve the photoelectric conversion characteristics of the flexible CIGS solar cell formed on the polymer film.
For the addition of Na, Non-Patent Documents 1 and 2 propose a method for depositing a sodalime glass between a Mo electrode layer and a substrate. However, when a sodalime glass is deposited as proposed in Non-Patent Documents, the number of manufacturing processes increases, resulting in a reduction in productivity.
Patent Document 2 proposes a method in which NaF is added to a Mo electrode layer so as to diffuse Na from the Mo electrode layer to the light absorbing layer. However, when NaF is added to the Mo electrode layer, a large amount of Na is concentrated between the Mo electrode layer and the substrate, so that interfacial separation between the Mo electrode layer and the substrate may occur.
Since it is difficult to control the amount of diffusion of Na from a glass substrate or the like, there is proposed a method for adding a Na compound directly to a light absorbing layer as a method for adding Na without Na diffusion. For example, Patent Document 3 proposes a method in which a diffusion barrier layer is disposed between a glass substrate and a light absorbing layer and then Na2S or Na2Se is added to the light absorbing layer while blocking diffusion of Na from the glass substrate so as to control the content of Na in the light absorbing layer.
In order to add a Na compound directly to a light absorbing layer, Patent Document 4 proposes a Cu—Ga sputtering target containing 0.05 to 1 at % of Na, wherein the Na is contained in the form of a NaF compound.
Here, Non-Patent Document 3 discloses a CIGS film to which 2% of Na is added, and thus, it has been found that a CIGS solar cell having a favorable power generation efficiency may be obtained even in the case of the addition of high concentration Na which has not been conventionally considered.